In reply to phil.

Normal browser behavior: request goes to server, server sends back response. This response includes links to other resources - fonts, css, JS, and images. The browser notices these resources, and sends additional requests to the server and fetches those resources.

What the new responsive image approach does is provide additional attributes to the image tag that alerts the browser to the different image files available for that particular image tag so that the browser can then intelligently request the right image file (source) for whatever window/viewport size or even resolution support the browser has. This means the browser can request the right "sized" image file for an image instead of being served an overly large image and resizing down to fit the space after the fact.